crypto: xilinx - Updated Makefile for xilinx subdirectory

This patch updates the Makefile for xilinx subdirectory.
CONFIG_CRYPTO_DEV_ZYNQMP_AES protects zynqmp-aes-gcm.o and it is used
twice (in drivers/crypto/Makefile and drivers/crypto/xilinx/Makefile)
and it is enough to use it once.

Signed-off-by: Harsha <harsha.harsha@xilinx.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>